The invention relates generally to the field of cycles, and in particular to bicycles. More specifically, the invention relates to an electric assist bicycle which is configured to maximize the efficiency of the motor and to prolong the life of the battery which supplies electrical current to the motor.
Over the last 150 years, the bicycle has evolved to become one of the most efficient means of transportation in terms of conversion of energy into distance travelled. For example, most modern bicycles require only about 400 watts (1/2 horsepower) to propel the bicycle at 15 m.p.h. on level ground. The efficiency of the bicycle has also been optimized to minimize the effort required by the rider. For instance, most modern bicycles include an efficient gear system to minimize rider effort.
To further reduce the amount of human effort required to propel a bicycle, a variety of electric bicycles have been introduced. Presently, about 50 to 100 companies are producing or are planning to produce electric bicycles. In most cases, however, such bicycles do not utilize the efficiency of the bicycle through the use of mechanical gears.
The human muscle and modern battery are similar in their ability to produce power from stored energy. Similarly, both are able to produce more energy by keeping the torque per stroke low and the frequency high.
The human muscle is able to function in two states: anaerobic or aerobic. In anaerobic contraction, the muscle utilizes stored ATP fuel to power the muscle without the need for oxygen. In this case, the muscle can produce large amounts of energy for a short duration. The byproduct of this high energy output is lactic acid. As muscle contraction continues in an anaerobic state, the lactic acid in the muscle builds until it inhibits further muscle contraction. After a period of rest, the lactic acid is removed from the muscle by the blood system and muscle contraction can continue (assuming a sufficient store of ATP fuel). Aerobic muscle contraction allows for extended periods of exertion, but at a lower level of power than anaerobic exercise. In aerobic exercise, sufficient oxygen is supplied to the muscle so that the muscle is able to use the soluble fat in the blood as the primary fuel.
The gears of modern bicycle allow the rider to exercise the muscle in the aerobic range to allow continuous long distance riding. The gears are utilized to keep the rider's pedal speed at a high rotating speed (usually between about 60 to 100 rpm). At higher pedaling speeds, the force output for muscle contraction is low so that the muscle is able to stay in the aerobic region.
The original bicycle used a single fixed gear ratio (similar to most electric bicycles) and was severely limited in its ability to negotiate steep terrain. The number of gears on a bicycle has evolved so that the present mountain bike has up to 27 gears to allow for riding on a variety of terrains.
Similar to the human muscle, the modern battery has an efficient and an inefficient region. The battery delivers current to the motor, which produces torque in the motor. The motor torque increases linearly with motor current. High currents are inefficient.
At high current discharge rates, the battery experiences problems similar to lactic acid buildup in the human muscle. More specifically, in the battery, hydrogen gas is formed on the charge plate. Hydrogen gas acts as a barrier to the transfer of electrons. As the high current discharge continues, the hydrogen continues to build on the plates until the battery is unable to deliver current.
Another important issue to consider at high current discharge rate is that the run time of the battery is reduced exponentially with linear increases in motor current. Further, motor thermal losses are experienced which increase with the square of the motor current. Hence, increased motor current wastes available energy two non-linear ways, i.e., battery losses and motor resistance losses.
As one example, a motor mounted directly to the rear wheel on the bicycle has only a fixed gear ratio. Hence, to obtain a four times increase in torque, the motor current must be increased by four times. However, the four times increase in the motor current increases motor resistive losses by 16 times and thus results in a significant loss in battery run time and reduction in motor efficiency.
The available power from the battery is an exponential function of the rate of current use. Hence, as current discharge increases, the available energy from the battery decreases exponentially. Hence, as more torque is required to move the bicycle (such as during hill climbing or acceleration), more current will be required, thereby exponentially decreasing the available power from the battery.
Hence, it would be desirable to provide improved electrically assisted bicycles and methods for their use which would overcome or greatly reduce these and other problems. The electric bicycles of the invention should be configured to maximize the efficiency of the motor, minimize current use, and thus maximize battery life. It would be desirable if such features could be accomplished by minimizing the required torque while keeping the rotational rate of the motor as high as possible. Preferably, the electric bicycles of the invention will employ the use of a gear system so that torque may be minimized, especially during hill climbing and acceleration. It would further be desirable if the electric bicycles of the invention provided for automatic shifting to keep the motor speed near maximum output while minimizing torque. In another aspect, it would be desirable if such electric bicycles were able to operate using either the motor or the pedals in a parallel manner. At the same time, it would be preferable if such electric bicycles employed the use of a motor which did not turn the crank arms. Such electric bicycles and methods should also be compatible with conventional bicycle equipment, such as derailleurs so that shifting may be accomplished with minimal modification to existing bicycles. Finally, it would be preferable to incorporate the batteries into the bicycle in a manner such that the overall appearance of the bicycle is aesthetically pleasing, such the batteries are protected, and such that the bicycle is provided with a low center of gravity.